The present inventions generally relate to radio communications and, more particularly, to multi-beam antennas utilized in cellular communication systems.
Cellular communication systems derive their name from the fact that areas of communication coverage are mapped into cells. Each such cell is provided with one or more antennas configured to provide two-way radio/RF communication with mobile subscribers geographically positioned within that given cell. One or more antennas may serve the cell, where multiple antennas commonly utilized are each configured to serve a sector of the cell. Typically, these plurality of sector antennas are configured on a tower, with the radiation beam(s) being generated by each antenna directed outwardly to serve the respective cell.
A common wireless communication network plan involves a base station serving three hexagonal shaped cells or sectors. This is often known as a three sector configuration. In a three sector configuration, a given base station antenna serves a 120° sector. Typically, a 65° Half Power Beamwidth (HPBW) antenna provides coverage for a 120° sector. Three of these 120° sectors provide 360° coverage. Other sectorization schemes may also be employed. For example, six, nine, and twelve sector sites have been proposed. Six sector sites may involve six directional base station antennas, each having a 33° HPBW antenna serving a 60° sector. In other proposed solutions, a single, multi-column array may be driven by a feed network to produce two or more beams from a single aperture. See, for example, U.S. Patent Pub. No. 20110205119, which is incorporated by reference.
Increasing the number of sectors increases system capacity because each antenna can service a smaller area. However, dividing a coverage area into smaller sectors has drawbacks because antennas covering narrow sectors generally have more radiating elements that are spaced wider than antennas covering wider sectors. For example, a typical 33° HPBW antenna is generally two times wider than a common 65° HPBW antenna. Thus, costs and space requirements increase as a cell is divided into a greater number of sectors.
To solve these problems, antennas have been developed using multi-beam forming networks (BFN) driving planar arrays of radiating elements, such as the Butler matrix. BFNs, however, have several potential disadvantages, including non-symmetrical beams and problems associated with port-to-port isolation, gain loss, and a narrow band. Classes of multi-beam antennas based on a classic Luneberg cylindrical lens (Henry Jasik: “Antenna Engineering Handbook”, McGraw-Hill, New York, 1961, p. 15-4) have tried to address these issues. And while these lenses can have better performance, the costs of the classic Luneberg lens (a multi-layer, cylindrical lens having different dielectric in each layer) is high and the process of production is extremely complicated. Additionally, these antenna systems still suffer from several problems, including beam width stability over the wide frequency band and high cross-polarization levels. Accordingly, there is a need for an antenna system that solves these problems to provide a high performance multi-beam base station antenna at an affordable cost.